Following the keywords, i would also suggest it goes into the weapon and armor section. There isn't much use to give alot of weapon and armor choice if no mechanics cover them. For example, a dwarf sword (p.433) is basically the same thing as a hand axe, a mace, a quarterstaff, a short sword, etc. I don't even understand why the price of the short sword is not the same as the hand axe (it has the exact same statics).
Simple keywords to help out. Like breaking a door (which as an armor) could have "vulnerable chopping" to reduce by half the door's armor against an axe. It would make alot more sense.
For the armor, you already have a keyword (living armor p.411), which has no mechanics (except for obsidiman restriction) associate with it. I personally use the keyword with the armor degradation rule (armor auto-repair itself). But that would not be enough if weapon gain keywords. That being said, why would anyone put an Obsidiman skin (p.435) ?? For 3/1, double de price of espagra cloak, the disavantage are huge. Even worst, for half the price, you get a better armor (Hide armor) without the "Kill on sight" status. You don't want the ini penalty, fine... get a Padded Leather for a 1/5 of the price without the penalty. Who would pay for an armor that would put any obsidiman against you? Basically, i've never seen anyone, except a poorly built ennemy npc, that would put an Obsidiman skin on.
That is... unless the obsidiman skin has a keyword that would balance the penalties. Making it "fire and cold resistant" for example would make it alot more common (unfortunatly for obsidiman). It would make sense to have an armor that someone would actually wear.
And on a final thought, keywords on armor should also consider armor stacking. It's been around for a while, and some mechanics in the 3rd allowed for example to stack Espagra Cloak (because... it's a cloak) on another set of armor. Can the Blood Pebble armor be stacked with a Padded Leather ? Even more historic, can you put some kind of leather or padded cloth under a chain mail (because chain mail was rarely put on naked). And if so, is the armor value the same? Well simple enough, I would suggest either or both 2 keywords. #1 "stackable" and #2 "improve"
#1 The keyword "stackable" would state in the mechanic that a stackable armor only provide half the armor value (rounded down) if it's stacked, and also add another point of initiative penalty. Let's make an example :
I'm wearing a Ring Mail. Since Padded Cloth has the "stackable" keyword, i can put the Ring Mail on the Padded Cloth for a protection of 7-0-3. Oh! it's exactly like a Chain Mail. Of course, the Chain Mail would cost like 70 more silvers for the same protection, but that's not considering the fact that the Chain Mail could also be stacked with the Padded Cloth. Now, if you think this can be a problem with high armor value, don't forget that some weapon could cut in half your armor value.
#2 The keyword "improve" would state in the mechanic that an improved armor doesn't provide more armor, but looses one vulnerability when it's combined, at a price of 1 initiative penalty. Let's make two examples :
I'm wearing a Chain Mail 7-0-3. The Chain Mail is "pierce vulnerable". By adding a Padded Cloth, the Chain Mail would be 7-0-4 without "pierce vulnerable".
I'm wearing a Plate Armor. The Plate Armor is "lightning vulnerable". By adding a Padded Cloth, the Plate Armor is no longer vulnerable, at a price of 1 more initiative penalty.
Adding keywords on weapons and armors would make the choice alot more critical, and also add the fun factor when you realize you choose the right weapon against your enemy.
Edit : I've made an entry in the Gamemaster section House rules to give a suggestion about this idea.
viewtopic.php?f=18&t=1528&p=15384#p15384